The present invention relates to plasma etching processes and, more particularly, to a method for monitoring and for terminating the etch process at the proper time based on the data gathered by measuring the RF voltage in the plasma reactor.
Major efforts in the semiconductor industry have been made to develop plasma etch processed to define fine line geometries in integrated circuit devices. Such processes typically occur in a reactive ion etch (RIE) reactor where the semiconductor device is placed in a chamber on an electrode, a reactive gas is introduced, and an electrical discharge alternating at an RF frequency is applied across the two electrodes in the reactor. This creates a highly reactive plasma which reacts with material on the semiconductor device, producing a volatile gas which can be pumped out of the chamber. The surface to be etched is generally masked by photoresist or other suitable material to define the fine line geometry.
While plasma etch processes have been shown to be useful in defining microminiature features, care must be exercised to control the reaction such that sufficient, yet not excessive etching takes place. If the etching process is ended prematurely the mask pattern will not be completely defined, and if the process continues too long, undercutting, excessive attack of underlying layers, and other problems can result. Nonuniformities in film thickness, variations in film properties which cause variation in etch rates, as well as changes in the plasma etch characteristics form wafer to wafer or between successive batches of wafers render the use of a fixed process time an inexact means for terminating the etch process.
Several approaches for determining endpoint have been described in the art. One method involves the use of laser interferometry on either a monitor wafer or a particular site on product wafers. The laser measures the thickness of the film removed as the etch process proceeds and endpoint is signaled after the film is etched away, plus some additional etch time afterwards. A disadvantage of this method is that frequently the film thickness of the monitor wafer or monitor site is not the same as that on the product chips on the wafer. This is due to the need for a uniform pattern area large enough for the laser to register. Further, the apparatus used for laser endpoint detection is not suitable for installation in all plasma reactors.
Another apporach is to detect changes in the optical emission spectra of the reaction and to stop the plasma etch process when a given spectral fingerprint is achieved. Although this method does reflect the bulk process across all of the wafers, it requires a complex system of filters, monochromators and photomultiplier tubes.
In U.S. Pat. No. 4,358,338 to Downey et al., the plasma etching endpoint is determined by measuring the changes in current at the surface of a workpiece.
U.S. Pat. No. 4,207,137 to Tretola discloses a method for monitoring the impedance of the plasma during the reaction. The impedance is measured in an asymmetric low pressure RIE reactor in which the wafers are loaded batch-wise on the lower electrode. The plasma is turned off once the impedance reaches a minimum or a maximum value and the monitor trace is essentially flat, i.e., it does not change with time, or some predetermined time thereafter. A substantial area of the lower electrode is not covered by the semiconductor wafers to be etched, and this area remains a relatively constant source of secondary electrons for the plasma. Thus, the plasma impedance will only be sensitive to rather large changes such as etching through an entire layer of material to expose a layer of different material. This method can not detect subtle changes in the material being etched and may not be sufficiently sensitive for the increasingly small geometries used in current integrated circuit technologies. Also, the measurement of impedance mismatch between the plasma and the RF source is somewhat difficult and complex.